Method and Device for Capacitive Near-Field Communication in Mobile Devices

ABSTRACT

A mobile device includes a conductive element and a ground node. The conductive element is configured to be detected by a proximity sensor. A switch is coupled between the conductive element and ground node. The conductive element is coupled to the ground node by closing the switch. A first memory element is configured to control the switch. The first memory element includes a register bit coupled to a control terminal of the switch. A data output is configured to control the switch. A FIFO is configured to provide data to the data output. The first memory element includes a FIFO. A capacitive touch controller is configured to measure a capacitance of the conductive element. A digital processing unit is configured to convert the capacitance of the conductive element to a bit of data. A second memory element is configured to store the bit of data.

FIELD OF THE INVENTION

The present invention relates in general to wireless communication and,more particularly, to a method and device for capacitive near-fieldcommunication in mobile devices.

BACKGROUND OF THE INVENTION

Smartphones and other mobile devices have rapidly become ubiquitousthroughout the world. Mobile phones and tablet computers are commonlyseen in use at restaurants, in waiting rooms, or on street corners.Mobile devices are used for gaming, photography, listening to music,social networking, or simply talking with another person via a built inmicrophone and speaker.

Mobile devices enrich lives by keeping family and friends incommunication, allowing any moment to be captured as a photo or video,and providing a means of contacting someone in an emergency situation.FIG. 1a illustrates a mobile device 10. Mobile device 10 is atouchscreen slate cellular (cell) phone. In other embodiments, mobiledevice 10 is a tablet computer, pager, GPS receiver, smartwatch or otherwearable computer, laptop computer, handheld game console, or any otherdevice capable of radio communication.

Mobile device 10 includes touchscreen 12 on a front side of the mobiledevice. Touchscreen 12 is used to display a graphical user interface(GUI). The GUI on touchscreen 12 presents feedback, notifications, andother information to a user as determined by an operating system ofmobile device 10. Touchscreen 12 is sensitive to physical touch frombody parts of a user of mobile device 10. Touchscreen 12 utilizesresistance, capacitance, acoustic waves, an infrared grid, opticalimaging, or other methods to determine the presence and location of auser's touch.

In one common usage scenario of mobile device 10, touchscreen 12displays a button as a part of the GUI, and a user touches the locationof the button on the touchscreen to perform an action associated withthe button. In one embodiment, touchscreen 12 displays a 3×4 telephonekeypad. A user dials a telephone number on the displayed keypad bytouching touchscreen 12 at the locations where the desired numbers todial are displayed. Touchscreen 12 displays an alphanumeric keyboardalong with, or as an alternative to, the telephone keypad, with a usertouching the touchscreen in the location of letters, numbers, or symbolsto be entered in a text input field displayed on the touchscreen.Touchscreen 12 is also used to watch downloaded or streamed videos, orplay games, with a user's touch controlling playback of the video orplay of the game. In some embodiments, touchscreen 12 is sensitive to auser's touch when the display component of the touchscreen is disabled.While listening to music, a user pauses the music, or advances to thenext track of music, by drawing a symbol on touchscreen 12 even thoughnothing is displayed on the touchscreen.

Buttons 14 provide an alternative user input mechanism to touchscreen12. Buttons 14 perform functionality depending on the programming of theoperating system running on mobile device 10. In one embodiment, buttons14 return the GUI on touchscreen 12 to a home screen, go back to aprevious GUI screen, or open up a menu on the GUI. In other embodiments,the functionality of buttons 14 changes based on a context displayed ontouchscreen 12.

Speaker 16 provides audible feedback to a user of mobile device 10. Whenmobile device 10 receives an incoming message, speaker 16 produces anaudible notification sound to alert a user to the received message. Anincoming telephone call causes a ringing sound from speaker 16 to alertthe user. In other embodiments, a musical ringtone, selectable via theGUI on touchscreen 12, is played via speaker 16 when an incomingtelephone call is received. When mobile device 10 is used to participatein a telephone call, a user of the mobile device speaks into microphone17 while the other conversation participants' voices are reproduced byspeaker 16. When a user watches a movie or plays a game, the soundassociated with the movie or game is produced by speaker 16 for the userto hear.

Front facing camera 18 provides visual feedback to the operating systemof mobile device 10. Camera 18 creates a digital image of the areafacing touchscreen 12. Camera 18 is used in video chat applicationsrunning on mobile device 10 to capture video of a user's face during aconversation. Mobile device 10 transmits the video of a user to anothermobile device in another location, and receives a streaming video ofanother person using the other mobile device which is displayed ontouchscreen 12. Camera 18 is also used to take selfies or otherpictures. When camera 18 is used to take pictures, touchscreen 12displays the image being captured by the camera so that the touchscreenis an electronic viewfinder. Captured photographs are stored on memorywithin mobile device 10 for subsequent viewing on touchscreen 12,sharing on social networks, or backing up to a personal computer.

Housing 20 provides structural support and protection for the internalcomponents of mobile device 10. Housing 20 is made of rigid plastic ormetallic materials to withstand environmental hazards which cause harmto the circuit boards and other components within mobile device 10 ifexposed directly. In one embodiment, a panel of housing 20 oppositetouchscreen 12 is removable to expose interchangeable parts of mobiledevice 10 such as a subscriber identification module (SIM) card, flashmemory card, or battery. Housing 20 includes a transparent glass orplastic portion over touchscreen 12, which protects the touchscreen fromenvironmental factors while allowing a user's touch to be sensed throughthe housing.

FIG. 1b illustrates a user 30 operating mobile device 10 as a telephone.A portion of housing 20 is removed to illustrate antenna 32 withinmobile device 10. User 30 holds mobile device 10 with speaker 16 over anear of the user. Microphone 17 is oriented toward a mouth of user 30.When user 30 speaks, microphone 17 detects and digitizes the user'svoice for transmission to a person the user is speaking with. The personthat user 30 is speaking with transmits a digitized voice signal tomobile device 10 which is reproduced on speaker 16 and heard by theuser. User 30 thereby converses with another person using mobile device10.

Mobile device 10 sends a voice signal of user 30, and receives a voicesignal of a person being conversed with, using a cellular network orother network capable of voice traffic. In various embodiments, mobiledevice 10 transmits voice signals and other data over Wi-Fi, Bluetooth,GSM, CDMA, LTE, HSPA+, WiMAX, or other wireless network types. Mobiledevice 10 transmits a voice signal using radio frequency (RF)electromagnetic waves emanating from RF antenna 32. An RF amplifier inmobile device 10 supplies an electric current, which contains the voiceinformation and oscillates at radio frequencies, to antenna 32. Antenna32 radiates energy of the current as electromagnetic waves through thearea surrounding mobile device 10. The electromagnetic waves reach acellular tower which forwards the voice signal on to ultimately bereceived by the person that user 30 is conversing with.

FIG. 1c is a block diagram of an RF section 33 of mobile device 10. RFsection 33 represents a portion of the circuitry located on a circuitboard within mobile device 10. RF section 33 includes microcontroller orcentral processing unit (CPU) 34, RF transceiver 36, RF amplifier 38,and antenna 32. For mobile device 10 to receive an audio signal or otherdigital data, radio waves are first received by antenna 32. Oscillatingelectric and magnetic fields of an incoming radio wave exert force onelectrons in antenna 32, causing the electrons to oscillate and creatinga current in the antenna. RF transceiver 36 demodulates the incomingsignal to eliminate RF frequencies and sends the underlying data to CPU34.

When mobile device 10 is transmitting data, CPU 34 first provides datato be transmitted. In one embodiment, CPU 34 receives audio data frommicrophone 17 and performs digital signal processing functions on theaudio data. CPU 34 performs any digital signal processing or basebandprocessing required for the audio data, or a separate digital signalprocessor (DSP) or baseband integrated circuit (IC) is used. In otherembodiments, non-voice data is sent, e.g., an outgoing text message or auniform resource locator (URL) of a website which user 30 wishes to viewon touchscreen 12. Once CPU 34 has received or generated the data to betransmitted, the data is sent from the CPU to RF transceiver 36. RFtransceiver 36 generates an RF signal containing the data to betransmitted by modulating the data using the frequency for a networkthat mobile device 10 is communicating with.

The RF signal is sent from RF transceiver 36 to RF amplifier 38. RFamplifier 38 amplifies the signal from RF transceiver 36 to generate ahigher power RF signal for transmission by antenna 32. RF amplifier 38sends the amplified RF signal to antenna 32. The amplified RF signalcauses an oscillating current of electrons within antenna 32. Theoscillating electric current creates an oscillating magnetic fieldaround antenna 32 and an oscillating electric field along the antenna.The time-varying electric and magnetic fields radiate away from antenna32 into the surrounding environment as an RF electromagnetic wave.

The output power of RF amplifier 38 is controlled by CPU 34. CPU 34controls the strength of an RF signal emanating from antenna 32 byconfiguring a gain setting of RF amplifier 38. A device receiving radiowaves from mobile device 10 can be from a few feet away for in-homeWi-Fi, to a few miles away for rural cellular service, or potentiallyeven further away from the mobile device. A higher gain setting of RFamplifier 38 causes a higher power electromagnetic radio wave to emanatefrom mobile device 10. A higher power electromagnetic radio wave isreceived at a location further away from mobile device 10.

Antenna 32 is omnidirectional, i.e., the antenna radiates energyapproximately equally in every direction from mobile device 10. Anomnidirectional antenna 32 gives mobile device 10 good connectivity witha cellular tower without regard to the angle the mobile device is heldat. However, due to the omnidirectional nature of antenna 32, asignificant amount of RF electromagnetic radiation from the antenna isradiated into user 30 when the user holds the mobile device near a bodypart, as illustrated in FIG. 1b . Some health concerns exist in relationto RF radiation from mobile devices, such as mobile device 10, beingabsorbed by the human body. Some studies suggest that RF energy absorbedby the body may be linked to cancer and other illnesses.

Specific absorption rate (SAR) is a measure of the rate at which energyis absorbed by the human body when exposed to an RF electromagneticfield. SAR measures exposure to electromagnetic fields between 100 kHzand 10 GHz. A SAR rating is commonly used in association with cellphones and magnetic resonance imaging (MRI) scanners.

When measuring SAR due to mobile device 10, the mobile device is placedat the head in a talk position, as illustrated in FIG. 1b . The SARvalue is then measured at the location that has the highest absorptionrate in the entire head, which is generally the closest portion of thehead to antenna 32. In the United States, the Federal CommunicationsCommission (FCC) requires that mobile devices have a SAR level at orbelow 1.6 watts per kilogram (W/kg) taken over the volume containing amass of 1 gram of tissue that is absorbing the most RF energy. InEurope, the European Committee for Electrotechnical Standardization(CENELEC) specifies a SAR limit of 2 W/kg averaged over the 10 grams oftissue absorbing the most RF energy.

Regulations limiting the SAR from mobile device 10 in effect limit theRF power of the mobile device when in use near the body of user 30.Limiting RF output limits signal strength and can degrade connectivityof mobile device 10 to cell phone towers. FIGS. 2a-2c show graphs of SARversus the distance of mobile device 10 from user 30. In FIGS. 2a and 2b, RF amplifier 38 has a constant power output. In FIG. 2a , CPU 34 hasconfigured RF amplifier 38 for high RF power and good connectivity ofmobile device 10 to cell phone towers. Line 40 illustrates that with aconstant RF power output, SAR is reduced as mobile device 10 is movedfurther away from user 30, i.e., further right on the graph in FIG. 2a .As mobile device 10 is moved closer to user 30, SAR increases.

Radiation emanating from mobile device 10 attenuates as the radiationtravels further away from antenna 32. When mobile device 10 is directlynext to the head of user 30, much of the radiation emanating fromantenna 32 is concentrated on a small area of the head, resulting in ahigh SAR. When mobile device 10 is further away from user 30, radiationspreads out and hits a larger area of the user's body at a lower energylevel. Much of the radiation which hits user 30 when mobile device 10 isheld up to the head will miss the user when the mobile device is held ata distance.

Line 40 shows that when configured for high RF power and goodconnectivity, mobile device 10 will exceed SAR regulatory limit 42 whenthe mobile device is held within a distance d of a body part of user 30.In one embodiment, the distance d at which mobile device 10 exceeds SARregulatory limit 42 when configured for high power output is 10millimeters (mm). Mobile device 10 as configured in FIG. 2a includesgood connectivity but is out of compliance with SAR regulations.

One solution to ensure that the SAR of mobile device 10 remains underregulatory limit 42 is to reduce the RF output power of the mobiledevice, illustrated by FIG. 2b . Line 44 shows that as mobile device 10is moved further away from user 30, SAR is reduced, as with theconfiguration of FIG. 2a . However, in FIG. 2b , mobile device 10 isconfigured for a lower RF output, and does not exceed SAR regulatorylimit 42 when the mobile device is held against user 30. The lower RFoutput makes mobile device 10 in compliance with SAR regulations, butreduces connectivity of the mobile device.

FIG. 2c illustrates another solution to maintaining the SAR of mobiledevice 10 under regulatory limit 42. When mobile device 10 is held at adistance greater than d from user 30, RF power output of the mobiledevice, illustrated by line 46, is at a level similar to the higherpower setting illustrated in FIG. 2a . When mobile device 10 is movedwithin a distance d of user 30, i.e., the distance at which SAR wouldexceed regulatory limit 42 in the configuration of FIG. 2a , the RFoutput of the mobile device is reduced to remain under the regulatorylimit. The reduced RF output within distance d is illustrated by line48, which is similar to line 44 of FIG. 2b . As configured in FIG. 2c ,mobile device 10 includes good connectivity when held a distance greaterthan d from user 30, and a reduced RF output to remain under SARregulatory limit 42 when held within a distance d of the user.

To implement the configuration illustrated in FIG. 2c , mobile device 10includes a proximity sensor used to detect distance from user 30. Whenthe proximity sensor detects user 30 is within a distance d of theproximity sensor, CPU 34 reduces the RF power output of RF amplifier 38to prevent the SAR of mobile device 10 from rising above regulatorylimit 42. When the proximity sensor detects no human body within adistance d of mobile device 10, CPU 34 increases RF power output toimprove connectivity.

FIG. 3a illustrates mobile device 10 with a portion of touchscreen 12removed to reveal near-field communication (NFC) loop antenna 50 and NFCchip 52. NFC loop antenna 50 is significantly smaller than thewavelength of the carrier signal used for communication over the NFCloop antenna. The short length of NFC loop antenna 50 compared to thewavelength of a carrier signal reduces the magnitude of electromagneticwaves radiating from the NFC loop antenna while still producingsignificant magnetic or electric fields near the loop antenna. Inductivecoupling between NFC loop antenna 50 of mobile device 10 and anotherdevice with an NFC loop antenna allows communication between the twodevices.

NFC technology is used, as illustrated in FIG. 3b , for communicationbetween two mobile devices, such as mobile device 10 and mobile device54. User 30 can transfer files or other data between mobile device 10and mobile device 54 by holding the mobile devices within a few inchesand initiating NFC communication. In some embodiments, NFC communicationis automatically triggered when two devices with NFC are within range ofeach other. When mobile device 54 is detected near NFC loop antenna 50of mobile device 10, touchscreen 12 displays various options user 30 canchoose from to utilize the NFC connection.

While NFC enables useful functionality of mobile device 10, NFC loopantenna 50 and NFC chip 52 add a significant cost to mobile device 10 interms of the area required for parts as well as manufacturing cost andcomplexity. NFC capability requires not only the addition of NFC loopantenna 50 and NFC chip 52, but a number of other parts such asresistors and capacitors are required in mobile device 10 to make NFCwork. In addition, transmitting over NFC loop antenna 50 draws asignificant amount of current, which drains battery power of mobiledevice 10.

One goal of mobile device manufacturers is to reduce the number of partsused in mobile devices. Reducing the number of parts in a mobile deviceallows the mobile device to be manufactured thinner and morelightweight, which are characteristics desired by today's consumers.Reducing the number of parts also reduces the cost and complexity ofmanufacturing mobile devices, which increases profits for manufacturersand reduces the price for consumers.

One way which manufacturers reduce the number of parts used to make amobile device is to use existing parts to add new functionality, or touse a single part for multiple functions which previously requiredmultiple parts. Challenges exist in combining functionality to use lessparts. Many times, a part is not useable for the multiple functions atone time, so the functions must be time multiplexed. Time multiplexingmeans that while a part is used for multiple functions, the part is onlyused for one of the functions at any given time. Another challenge tocombining functionality to use less parts is adapting existingtechnology to new uses. In some cases, the modifications necessary touse an existing part for a new function are not obvious, or requirechanges to integrated circuit modules.

SUMMARY OF THE INVENTION

A need exists to enable communication between a mobile device and othernearby devices without adding parts and complex circuitry to the mobiledevice. Accordingly, in one embodiment, the present invention is amethod of making a mobile device comprising the steps of providing aconductive element, providing a ground node, providing a switch coupledbetween the conductive element and ground node, providing a first memoryelement configured to control the switch, and providing a capacitivetouch controller configured to measure a capacitance of the conductiveelement.

In another embodiment, the present invention is a method of making amobile device comprising the steps of providing a conductive element,providing a ground node, providing a switch coupled between theconductive element and ground node, and providing a first memory elementconfigured to control the switch.

In another embodiment, the present invention is a method of making amobile device comprising the steps of providing a conductive element,providing a ground node, and providing a switch coupled between theconductive element and ground node.

In another embodiment, the present invention is a mobile devicecomprising a conductive element and a ground node. A switch is coupledbetween the conductive element and ground node.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c illustrate a mobile device with RF transmission capability;

FIGS. 2a-2c illustrate SAR versus distance from a human body for amobile device with and without the use of a proximity sensor;

FIGS. 3a-3b illustrate a mobile device including inductive near-fieldcommunication capability;

FIGS. 4a-4b illustrate a mobile device including a proximity sensor;

FIGS. 5a-5c illustrate electric fields between a proximity sensingelement, surrounding shielding areas, and a human finger;

FIGS. 6a-6c illustrate electric fields between a proximity sensingelement of a first mobile device and a proximity sensing element of asecond mobile device;

FIGS. 7a-7b illustrate a capacitive touch controller including thecapability to transmit data using a sensing element;

FIGS. 8a-8b illustrate transmitting data between two mobile devicesusing capacitive NFC; and

FIGS. 9a-9b illustrate FIFOs used to transmit and receive data overcapacitive NFC.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention is described in one or more embodiments in thefollowing description with reference to the figures, in which likenumerals represent the same or similar elements. While the invention isdescribed in terms of the best mode for achieving the invention'sobjectives, it will be appreciated by those skilled in the art that itis intended to cover alternatives, modifications, and equivalents as maybe included within the spirit and scope of the invention as defined bythe appended claims and their equivalents as supported by the followingdisclosure and drawings.

FIG. 4a illustrates mobile device 10 with a portion of touchscreen 12and housing 20 removed to reveal printed circuit board (PCB) 55 with CPU34, capacitive touch controller 56, sensing element 58, and shieldingarea 60 formed or disposed on the PCB. In other embodiments, a flexibleprinted circuit (FPC) is used instead of PCB 55. Capacitive touchcontroller 56 and sensing element 58 form a proximity sensor for mobiledevice 10. In some embodiments, proximity sensors are used which do notrequire a separate sensing element. Conductive trace 62 connects sensingelement 58 to capacitive touch controller 56, while conductive trace 64connects shielding area 60 to the capacitive touch controller.Conductive traces 66 provide communication between CPU 34 and capacitivetouch controller 56.

PCB 55 provides a base for mounting the electronic parts and forming theconductive traces necessary to provide the functionality of mobiledevice 10. PCB 55 includes other circuit elements and semiconductorpackages not illustrated as required to implement the functionality ofmobile device 10. PCB 55 includes all the electronic parts necessary formobile device 10. In other embodiments, the electronic parts for mobiledevice 10 are split across multiple PCBs. PCB 55 includes additionalparts such as a Universal Serial Bus (USB) port, random access memory(RAM), flash memory, a graphics processing unit (GPU), or a system on achip (SoC).

Capacitive touch controller 56 is an IC designed to measure theself-capacitance, or inherent capacitance, of sensing element 58.Self-capacitance is a capacitance measured between a conductive element,e.g., sensing element 58, and a ground potential. When the conductiveobject to be detected, e.g., a lap, finger, palm, or face of user 30, isnot present near sensing element 58, the self-capacitance of the sensingelement, Csensor, is the environmental capacitance, Cenv. Cenv isdetermined by electric fields from sensing element 58 interacting withthe environment near the sensing element. In particular, electric fieldsfrom sensing element 58 interact with nearby conductive material such asshielding area 60, traces 62-66, power and ground planes, conductivevias, and ICs. When a body part of user 30 is present near sensingelement 58, the self-capacitance of the sensing element, Csensor, isCenv plus the capacitance attributable to the body part, Cuser.Capacitive touch controller 56 is calibrated with the value of Cenv, andsubtracts Cenv from the total self-capacitance of sensing element 58.The remaining capacitance is the self-capacitance attributable to a bodypart of user 30 in proximity of sensing element 58, i.e., Cuser. Inpractice, a configurable capacitor bank within capacitive touchcontroller 56 cancels or counteracts the Cenv contribution toself-capacitance, leaving Cuser to be measured, although other methodsof isolating Cuser from Csensor are used in other embodiments.

If Cuser, i.e., the measured self-capacitance of sensing element 58attributable to user 30, is approximately equal to zero, capacitivetouch controller 56 reports to CPU 34 a lack, or absence, of proximityvia a memory mapped flag, as well as an interrupt. If Cuser is over athreshold associated with a human body part, capacitive touch controller56 reports proximity in a similar manner. In addition to a flagindicating proximity or lack thereof, capacitive touch controller 56reports to CPU 34 a digital value proportional to Cuser for each Cusermeasurement, whether proximity is detected or not. CPU 34 uses theproximity flag for simple applications where only proximity or lack ofproximity is needed, and uses the digital Cuser value to implementfunctionality that is more advanced.

Capacitive touch controller 56 senses self-capacitance of sensingelement 58 by first using a bank of capacitors to cancel Cenv, and thenconverting the remaining capacitance, Cuser, to a proportional voltagepotential. In some embodiments, the entire self-capacitance of sensingelement 58, Csensor, is converted to a proportional voltage and thenreduced by a voltage proportional to Cenv. The resulting voltage,proportional to Cuser, is converted to a digital value using an analogto digital converter. The digital Cuser value is processed to determinewhether Cuser exceeds a threshold for acknowledging proximity.

Sensing element 58 is a square of copper formed on a surface of PCB 55,although other shapes and other conductive materials are used for thesensing element in other embodiments. In one embodiment, the samephysical element is used for both antenna 32 and sensing element 58. Inembodiments with a single physical element used for antenna 32 andsensing element 58, a capacitor and inductor are used to filter RFsignals from reaching capacitive touch controller 56 and filter lowerfrequency signals from reaching RF amplifier 38 and RF transceiver 36.In other embodiments, any conductive element is used for sensing element58.

Sensing element 58 interacts with nearby conductive material, such asconductive traces, vias, and ground planes, as well as a lap, finger,palm, or face of user 30, via electric fields. When a charge is appliedto sensing element 58, an opposite charge is attracted toward thesensing element within any nearby conductive material. When the amountof conductive material near sensing element 58 is increased, a greateramount of electric charge is attracted to the sensing element for agiven voltage potential of the sensing element. Thus, theself-capacitance of sensing element 58 is a function of the amount ofconductive material near the sensing element. Conductive material havinga conduction path to a circuit node at a ground potential has a greatereffect on self-capacitance because the ground node provides a source ofadditional charge into the conductive material. Sensing element 58attracts opposite charge into nearby conductive material through theground node.

Shielding area 60 provides an electromagnetic shield substantiallysurrounding sensing element 58. A shielding area formed on a surface ofPCB 55 opposite sensing element 58 and shielding area 60 reduces theability of the sensing element to detect conductive material on a backside of mobile device 10.

Shielding area 60 is electrically connected to capacitive touchcontroller 56. Capacitive touch controller 56 drives shielding area 60with a similar voltage potential as sensing element 58 when measuringself-capacitance of the sensing element. In other embodiments, shieldingarea 60 is electrically connected to a ground potential. Connectingshielding area 60 to ground potential provides an increase to the Cenvcomponent of self-capacitance of sensing element 58. A higher Cenvrequires a larger capacitor bank within capacitive touch controller 56to counteract the higher Cenv. Driving shielding area 60 with a similarvoltage potential as sensing element 58 provides a lower Cenv, andreduces the required size of the capacitor bank within capacitive touchcontroller 56.

Conductive trace 62 connects sensing element 58 to capacitive touchcontroller 56. In some embodiments, multiple sensing elements are used,with each sensing element separately connected to capacitive touchcontroller 56 with a different conductive trace. In one embodiment,sensing elements are used to implement buttons 14, with the buttonsbeing activated when proximity of user 30 is sensed on a button.Capacitive touch controller 56 manipulates the voltage of sensingelement 58 and detects the self-capacitance of the sensing element viaconductive trace 62. Conductive trace 64 connects shielding area 60 tocapacitive touch controller 56. Capacitive touch controller 56 controlsthe voltage of shielding area 60 to be approximately equal to thevoltage of sensing element 58 via conductive trace 64.

Conductive traces 66 connect CPU 34 to capacitive touch controller 56.Traces 66 include lines for reset, interrupt, data, address, clock,enable, and other signals necessary for communication between CPU 34 andcapacitive touch controller 56. In one embodiment, CPU 34 communicateswith capacitive touch controller 56 using the inter-integrated circuit(I²C) protocol. Other communication protocols are used in otherembodiments.

Some functions of capacitive touch controller 56 are controlled by CPU34 using a single conductive trace 66 connected to a pin on thecapacitive touch controller, such as enabling or disabling sensing.Other functionality is exercised by CPU 34 reading from or writing tohardware registers within capacitive touch controller 56. A raw Cuservalue is read from a memory mapped hardware register internal tocapacitive touch controller 56. A register is also used by CPU 34 to setthe threshold value of Cuser when capacitive touch controller 56 reportsproximity. Some functionality is implemented with a discrete input oroutput pin on capacitive touch controller 56, as well as a hardwareregister within the capacitive touch controller. Capacitive touchcontroller 56 is reset by CPU 34 toggling a reset input pin of thecapacitive touch controller, or by the CPU writing to a soft resetregister within the capacitive touch controller.

In FIG. 4b , user 30 holds mobile device 10 up to his or her head. Theself-capacitance of sensing element 58 increases due to the interactionof electric fields between the sensing element and user 30. Prior tomobile device 10 being disposed in proximity to user 30, the area infront of the mobile device is occupied by air, which has a smallereffect on self-capacitance than the head of the user. Capacitive touchcontroller 56 detects the rise in self-capacitance of sensing element58, and notifies CPU 34 of the proximity of user 30. CPU 34 reduces thepower output of RF amplifier 38 accordingly so that mobile device 10remains in compliance with SAR regulations.

FIG. 5a is a partial cross-section of PCB 55 illustrating sensingelement 58 and optional shielding area 60 formed on a top surface of thePCB. An optional shielding area 70 is formed on a bottom surface of PCB55 opposite sensing element 58 and shielding area 60. An optionaloverlay 72 is formed over sensing element 58 and shielding area 60 forphysical isolation and protection of the sensing element and shieldingarea.

PCB 55 is formed from one or more layers of polytetrafluoroethylenepre-impregnated (prepreg), FR-4, FR-1, CEM-1, or CEM-3 with acombination of phenolic cotton paper, epoxy, resin, woven glass, matteglass, polyester, and other reinforcement fibers or fabrics. Electroniccomponents necessary for the functionality of mobile device 10, such asconductive traces and ICs, are formed or disposed on surfaces of PCB 55.In one embodiment, a multilayer PCB 55 is used which includes electroniccomponents on layers between a top and bottom surface of the PCB.Components at different layers of PCB 55 are connected by conductivevias formed in the PCB.

Sensing element 58 and shielding area 60, as well as traces 62-66 areformed as a layer of metal on PCB 55. In one embodiment, sensing element58, shielding area 60, and traces 62-66 are formed from a single uniformlayer of metal using subtractive methods such as silk screen printing,photoengraving, or PCB milling. In other embodiments, an additive orsemi-additive method such as physical vapor deposition (PVD), chemicalvapor deposition (CVD), electrolytic plating, electroless plating, oranother suitable metal deposition process is used. Shielding area 70 isformed from a similar process as sensing element 58 and shielding area60.

Sensing element 58, shielding area 60, shielding area 70, and conductivetraces 62-66 include one or more layers of aluminum (Al), copper (Cu),tin (Sn), nickel (Ni), gold (Au), silver (Ag), indium tin oxide (ITO),printed conductive ink, or other suitable electrically conductivematerial. Traces 62-66 are formed on the same surface of PCB 55 assensing element 58. In other embodiments, traces 62-66 are formed on thesurface of PCB 55 opposite sensing element 58, or on an intermediatelayer when a multilayer PCB is used. Conductive vias are used to connecttraces 62-66 to CPU 34, capacitive touch controller 56, sensing element58, and shielding area 60 when the traces are not formed on the samesurface as sensing element 58. A conductive via connects shielding area60 to shielding area 70 so that capacitive touch controller 56 drivesboth shielding areas to a similar voltage potential.

Shielding areas 60 and 70 provide a noise blocking function, as well asdirectionality for sensing element 58. Shielding areas 60 and 70 providean electromagnetic shield substantially surrounding sensing element 58in each direction other than the direction which sensing is desired.Electric fields from sensing element 58 interact with shielding areas 60and 70, which have a stable effect on self-capacitance, instead of otherobjects opposite the shielding areas which have a dynamic capacitancewith respect to the sensing element. Shielding areas 60 and 70 alsoreduce electromagnetic noise which impacts the accuracy of the detectedcapacitance.

With shielding areas 60 and 70 surrounding sensing element 58 on thebottom and sides, as illustrated in FIGS. 5a-5c , a body part isdetected when disposed over sensing element 58 opposite shielding area70. Shielding area 70 limits the detection capability of capacitivetouch controller 56 when a finger or other body part of user 30 isdisposed on the back side of PCB 55, i.e., on the opposite side of thePCB from sensing element 58. In some embodiments with an omnidirectionalantenna 32, shielding area 70 is not used so that a body part of user 30is detected whether the body part is on a front or back side of mobiledevice 10. Without shielding area 70, proximity is detected, and CPU 34reduces RF power output of mobile device 10, when a body part of user 30is within proximity on the back side of mobile device 10, e.g., the usersets the mobile device in his or her lap.

Shielding areas 60 and 70 are electrically connected to capacitive touchcontroller 56. Capacitive touch controller 56 drives shielding areas 60and 70 to a similar voltage potential as sensing element 58 when sensingself-capacitance of the sensing element. In other embodiments, shieldingareas 60 and 70 are electrically connected to a ground potential.Connecting shielding areas 60 and 70 to ground potential provides anincrease to the Cenv component of self-capacitance of sensing element 58due to ground providing a source of charges attracted to the sensingelement. A higher Cenv requires a larger capacitor bank withincapacitive touch controller 56 to counteract the higher Cenv. Capacitivetouch controller 56 driving shielding areas 60 and 70 to a similarvoltage potential as sensing element 58 reduces self-capacitance ofsensing element 58 by reducing the amount of charge the sensing elementattracts in the shielding areas.

Overlay 72 provides physical isolation and protection for sensingelement 58. Overlay 72 increases the robustness of mobile device 10 byprotecting sensing element 58 from environmental hazards such as dust,dirt, rain, and wind. In one embodiment, overlay 72 is a sheet ofplastic or glass integrated into housing 20. Overlay 72 is translucent,transparent, or opaque. Overlay 72 is formed from a material with anelectric field permittivity sufficient to allow electric fields topropagate between sensing element 58 and a body part of user 30 disposedin proximity to the sensing element.

FIG. 5b illustrates electric fields between sensing element 58 andshielding areas 60 and 70 when no human body part is in proximity of thesensing element. Electric fields 80 extend between sensing element 58and shielding area 60. Electric fields 82 extend between sensing element58 and shielding area 70. Electric fields 80 and 82 are simplifiedillustrations of the electric fields interacting with sensing element58. In practice, the electric fields are complex and extend not only toshielding areas 60 and 70, but also to any conductive material, such asconductive vias or conductive traces, near sensing element 58. Theenvironmental self-capacitance, Cenv, of sensing element 58 is a measureof electric fields 80 and 82 from the sensing element interacting withshielding areas 60 and 70 and other conductive material in proximity tothe sensing element when user 30 is not in proximity.

When a charge exists on sensing element 58, electric fields 80 and 82attract an opposite charge within shielding areas 60 and 70 toward thesensing element. A negative charge exists when there is an excess ofelectrons in the atoms of an object compared to the number of protons. Apositive charge exists when there is a deficit of electrons compared tothe number of protons. Negatively charged material attracts positivecharge, and positively charged material attracts negative charge. When afirst object has a positive charge, electrons in nearby conductiveobjects are attracted to the first object, creating an area of negativecharge in the nearby objects. When a first object has a negative charge,electrons in nearby conductive objects are repelled, creating an area ofpositive charge in the nearby objects. A negative charge and a positivecharge are opposites.

In FIG. 5c , finger 84 of user 30 is in the proximity of sensing element58. While a finger is illustrated, a lap, palm, face, or otherconductive object is also capable of being detected. Electric fields 86attract a charge to the tip of finger 84 that is opposite of a charge onsensing element 58. The charge attracted in finger 84 raises the totalamount of charge that must be supplied to sensing element 58 bycapacitive touch controller 56 to reach a given voltage potential of thesensing element. As charge per volt is a formula defining capacitance,additional conductive material with additional charge attracted tosensing element 58 raises the self-capacitance of the sensing element.In FIG. 4c , Cenv is represented by electric fields 80 and 82, Cuser isrepresented by electric fields 86, and Csensor is the sum of Cenv andCuser.

Capacitive touch controller 56 measures that the self-capacitance ofsensing element 58, and thus Cuser, has risen. A flag is set within ahardware register of capacitive touch controller 56, and the capacitivetouch controller asserts an interrupt signal to CPU 34. CPU 34 receivesthe interrupt and executes program code associated with a new proximityreading. In the case of mobile device 10, CPU 34 executes code whichreduces RF power output of RF amplifier 38 to prevent exceeding SARregulatory limit 42. In other embodiments, where capacitive sensing isused to implement buttons 14, CPU 34 executes program code associatedwith the pressing of a button when proximity is sensed.

Capacitive touch controller 56 uses sensing element 58 to detectproximity of conductive objects other than body parts of user 30. FIG.6a illustrates sensing element 88 of mobile device 54 placed inproximity to sensing element 58 of mobile device 10. Electric fields 86interact with sensing element 88, and capacitive touch controller 56detects that the self-capacitance of sensing element 58 has changed. Ascapacitive touch controller 56 changes the voltage potential of sensingelement 58, an opposite charge is attracted in sensing element 88.

The effect of sensing element 88 on the self-capacitance of sensingelement 58 is detected by capacitive touch controller 56 as Cuser.Mobile device 54 is able to control the electrical connectivity ofsensing element 88, thereby changing the effect of sensing element 88 onthe self-capacitance of sensing element 58. By repeatedly changing theeffect of sensing element 88 on the self-capacitance of sensing element58, mobile device 54 transmits a digital signal to mobile device 10.Mobile device 10 repeatedly senses the self-capacitance of sensingelement 58 to receive the digital signal from mobile device 54.

FIGS. 6b and 6c illustrate mobile device 54 modifying the effect ofsensing element 88 on the self-capacitance of sensing element 58 totransmit a binary data signal from mobile device 54 to mobile device 10.In FIG. 6b , mobile device 54 has coupled sensing element 88 to groundnode 90. Mobile device 54 includes a capacitive touch controller whichprovides the connection between sensing element 88 and ground node 90.Ground node 90 is a source of charge for sensing element 88. Ascapacitive touch controller 56 modifies the voltage potential of sensingelement 58, an opposite charge is attracted to sensing element 88 viaground node 90. Opposite charge attracted to sensing element 88 throughground node 90 requires that more charge be supplied to sensing element58 by capacitive touch controller 56 to reach a given voltage potential.Capacitive touch controller 56 recognizes the additional charge suppliedto sensing element 58 to reach a given voltage potential as an increasein the Cuser component of self-capacitance. Sensing element 88 connectedto ground node 90 has a similar effect on the self-capacitance ofsensing element 58 as finger 84 or another body part of user 30.

In FIG. 6c , mobile device 54 has disconnected sensing element 88 fromground node 90 using switch 92. Mobile device 54 puts sensing element 88in a high impedance, or hi-Z, state by disconnecting sensing element 88from ground node 90 and other sources of charge. Being in a highimpedance state means that the amount of current which flows to or fromsensing element 88 is significantly reduced. Switch 92 is formed fromone or more transistors within a capacitive touch controller of mobiledevice 54 operating as a switch. In other embodiments, any method ofswitching a connection of sensing element 88 between ground and highimpedance is used.

Switch 92 reduces the flow of charge to and from sensing element 88. Ascapacitive touch controller 56 changes the voltage potential of sensingelement 58, a significant opposite charge is not attracted into sensingelement 88. Mobile device 54 does not provide a source of significantcharge into sensing element 88 because switch 92 has disconnectedsensing element 88 from ground node 90. As the charge on sensing element58 changes, any charge already on sensing element 88 is moved aroundwithin sensing element 88. However, significant additional charge is notattracted onto sensing element 88 because switch 92 has cut off thesupply of charge, i.e., ground node 90. Without additional oppositecharge attracted into sensing element 88, the effect of sensing element88 on the self-capacitance of sensing element 58 is reduced. Capacitivetouch controller 56 does not detect a significant increase in Cuser fromthe proximity of sensing element 88. Opening switch 92 to put sensingelement 88 in a high impedance state has a similar effect on theself-capacitance of sensing element 58 as removing finger 84 or anotherbody part of user 30 from the proximity of sensing element 58. Closingswitch 92 again to couple sensing element 88 to ground node 90, asillustrated in FIG. 6b , has a similar effect as moving a body part ofuser 30 back into proximity of sensing element 58.

By successively switching sensing element 88 between being connected toground node 90 and high-impedance, mobile device 54 transmits a digitaldata signal to mobile device 10. Mobile device 10 detects theself-capacitance of sensing element 58 at least as fast as mobile device54 is changing the self-capacitance of sensing element 58 to receive thedigital data signal.

The connection between mobile device 10 and mobile device 54 usingsensing elements 58 and 88 is a serial communication link. Variousprotocols, similar to protocols known in the art, are used to transmitdata via the serial data connection. Synchronous or asynchronous datatransfer modes are used. In one embodiment, a connection of sensingelement 88 to ground node 90 corresponds to a logic ‘1’ value whileplacing sensing element 88 in a high impedance state corresponds to alogic ‘0’ value. Other forms of encoding digital data are used in otherembodiments. Various embodiments use start bits, stop bits, parity bits,and data bits in any number and combination to transmit a serial datapacket.

Capacitive touch controller 56 includes the capability to connectsensing element 58 to a ground node or to high impedance. Mobile device54 includes a capacitive touch controller 56 capable of measuring theself-capacitance of sensing element 88. To transmit digital data frommobile device 10 to mobile device 54, sensing element 58 is successivelyconnected between a ground node and high impedance while mobile device54 measures the self-capacitance of sensing element 88. Mobile device 10transmits data to mobile device 54 using similar protocols as used bymobile device 54 to transmit to mobile device 10.

Two-way communication is possible by time multiplexing the direction oftransmission between mobile device 54 and mobile device 10. Protocolssimilar to other half-duplex communication methods are used to controlthe direction of communication. In some embodiments, each mobile deviceincludes two sensing elements used for capacitive NFC which arepositioned such that the transmit pad of one mobile device is in acorresponding location to the receive pad on a second mobile device.When mobile device 10 and mobile device 54 are placed face-to-face orback-to-back, mobile device 10 includes a transmitting sensing elementin proximity to a receiving sensing element of mobile device 54, andmobile device 10 includes a receiving sensing element in proximity to atransmitting sensing element of mobile device 10.

FIG. 7a is a block diagram of internal components of capacitive touchcontroller 56. Analog front-end (AFE) 102 detects the self-capacitanceof sensing element 58 and outputs a digital value of theself-capacitance to digital processing unit 104. When data is beingreceived via capacitive NFC, the proximity readings are used to receivethe data. Registers 106 contain various hardware registers used bycapacitive touch controller 56 to report information to CPU 34, or bythe CPU to configure the capacitive touch controller. Data output 108 isused during transmission of data by capacitive touch controller 56 overcapacitive NFC. Data output 108 connects sensing element 58 to a groundpotential or to a high impedance based on the data to be transmitted.Multiplexer (MUX) or switch 110 couples sensing element 58 to dataoutput 108 when capacitive touch controller 56 is transmitting data toanother mobile device via capacitive NFC, and otherwise couples thesensing element to AFE 102 for sensing proximity of user 30 or receivingdata via capacitive NFC.

AFE 102 includes a configurable bank of capacitors which are adjusted toapproximately cancel the effect of Cenv so that capacitance due to theproximity of external conductive material, Cuser, is isolated andaccurately measured. Cuser is the portion of self-capacitance of sensingelement 58 attributable to a body part of user 30 when sensing element58 is used to detect proximity of a user. Cuser is the portion ofself-capacitance of sensing element 58 attributable to the sensingelement of another mobile device when mobile device 10 is receiving datavia capacitive NFC.

A digital value from registers 106 configures the bank of capacitors inAFE 102 based on a prior reading of Cenv. The configurable capacitorbank in AFE 102 is used to generate a voltage approximately proportionalto a previously detected Cenv. AFE 102 also generates a voltageapproximately proportional to Csensor, i.e., the total self-capacitanceof sensing element 58. AFE 102 subtracts the voltage proportional toCenv from the voltage proportional to Csensor to produce a voltageapproximately proportional to Cuser. The voltage proportional to Cuseris converted to a digital value by an analog-to-digital converter withinAFE 102 and output to digital processing unit 104. Cuser, Csensor, andCenv are each different values of the capacitance of the proximitysensor formed from capacitive touch controller 56 and sensing element58.

Digital processing unit 104 receives a digital value approximatelyproportional to Cuser from AFE 102 and writes the value to a hardwareregister in registers 106. The digital Cuser value written to a registerin registers 106 is available to CPU 34 by reading the register. Adifferent digital value, stored in a hardware register of registers 106and configured by CPU 34, indicates a threshold Cuser must reach inorder for capacitive touch controller 56 to report proximity to CPU 34.If the digital Cuser value from AFE 102 exceeds the threshold value fromregisters 106, digital processing unit 104 causes a proximity statusflag in registers 106 to become a logic ‘1’, and CPU 34 is interruptedfor handling of the proximity event.

Digital processing unit 104 stores a digital value of Cuser in registers106 each time the self-capacitance of antenna 32 is converted to a newCuser value. In one embodiment, digital processing unit 104 stores theraw Cuser value from AFE 102 in registers 106. In other embodiments,digital processing unit 104 adjusts the Cuser value before storage inregisters 106, e.g., by adjusting Cuser for drift of Cenv or byfiltering high frequency noise.

Registers 106 contain various memory mapped hardware registers used byCPU 34 to configure capacitive touch controller 56, or by the capacitivetouch controller to report proximity and other information to the CPU.Some hardware registers of registers 106 are set by a manufacturer forconfiguration aspects which the manufacturer desires to set permanentlyfor the lifetime of mobile device 10, or until modified by amanufacturer's update. Registers 106 include interrupt request (IRQ)bits used to notify CPU 34 when the proximity status of user 30 haschanged, i.e., the user has entered or left the proximity of antenna 32.Registers 106 also include IRQ bits for completion of a new reading ofCuser or a new calibration of Cenv. Registers 106 are used by CPU 34 toset a threshold value of Cuser when proximity is considered detected, toreset capacitive touch controller 56, and to set a frequency at whichperiodic capacitance readings are to occur, among other uses.

Registers 106 include read-only registers set by a manufacturer ofmobile device 10. One read-only register is used to store a referenceCenv reading, which the manufacturer calculates with mobile device 10 ina known state. The reference Cenv reading is used to verify subsequentCenv readings were validly made without user 30 in proximity, and isalso used to detect proximity when no other value of Cenv is available.Another read-only register in registers 106 is used to store atemperature coefficient of the self-capacitance of antenna 32. Theself-capacitance of antenna 32 has an approximately linear relationshipwith the temperature of mobile device 10. A coefficient defining therelationship between the self-capacitance of antenna 32 and thetemperature of mobile device 10 is stored in registers 106 for use bydigital processing unit 104 to accurately adjust Cuser readings toaccount for temperature changes.

Registers 106 include a status bit indicating whether the most recentmeasurement of Cuser indicated that user 30 was in proximity. CPU 34reads the status bit at any point to determine the proximity status ofuser 30. A logic ‘0’ indicates user 30 is not in proximity of mobiledevice 10, and a logic ‘1’ indicates the user is in proximity. In someembodiments, when in capacitive NFC mode, CPU 34 reads the status bit inregisters 106 to determine the latest bit of data from another device.In other embodiments, registers 106 contains a data register or FIFO forreceived data, and CPU 34 reads one or more bytes of data out ofregisters 106 at a time.

A FIFO, which is an acronym for first in first out, is a memory devicewhich holds multiple bytes or words of data. As data is received a bitat a time by sensing proximity, data processing unit 104 collects thebits into bytes or words and stores the data in the FIFO. CPU 34 isnotified that data is available in the FIFO, while capacitive touchcontroller 56 continues to receive data via capacitive NFC and store thedata in the FIFO. CPU 34 reads the received data from the FIFO in theorder which data processing unit 104 writes the data into the FIFO untilthe FIFO is empty of data.

Data output 108 includes a switch which connects sensing element 58 to aground node or puts the sensing element into a high impedance state. Theswitch in data output 108 controls whether sensing element 58 will bedetected by another nearby sensing element. When data output 108connects sensing element 58 to a ground node, the sensing element isdetected by another device. When data output 108 puts sensing element 58in a high impedance state, the sensing element is not detected by theother device.

Data output 108 reads or receives data to be transmitted over capacitiveNFC from registers 106. In one embodiment, data output 108 receives asingle bit at a time and toggles the connection between sensing element58 and a ground node accordingly. In other embodiments, data output 108handles an entire byte or word at a time. A multi-bit bus transfers datato be transmitted into data output 108, and data output 108 transferseach bit one at a time by switching the connection between sensingelement 58 and ground accordingly.

MUX 110 switches sensing element 58 between being coupled to AFE 102 andbeing coupled to data output 108. If mobile device 10 is transmittingdata to another device over capacitive NFC, MUX 110 connects sensingelement 58 to data output 108. AFE 102 is disconnected from sensingelement 58. If mobile device 10 is receiving data from another devicevia capacitive NFC, or detecting proximity of user 30, MUX 110 connectsAFE 102 to sensing element 58 for measurement of self-capacitance. Dataoutput 108 is disconnected from sensing element 58. MUX 110 is not usedin embodiments with separate sensing elements for transmitting andreceiving data. AFE 102 is connected to one sensing element, while dataoutput 108 is connected to another sensing element.

FIG. 7b illustrates data output 108 in detail. Data control 120 receivesdata from registers 106 which is to be transmitted via capacitive NFC.Switch 122 connects sensing element 58 to ground node 124, ordisconnects the sensing element from the ground node so that the sensingelement is in a high impedance state.

In one embodiment, data control 120 gets one bit at a time fromregisters 106. If the bit from registers 106 is a logic ‘1’, datacontrol 120 closes switch 122 so that sensing element 58 is detected byanother mobile device's proximity sensor, thereby communicating a logic‘1’ to the other mobile device. If the bit from registers 106 is a logic‘0’, data control 120 opens switch 122 so that sensing element 58 is notconnected to ground node 124. Sensing element 58 is at high impedanceand is not detected by another mobile device's proximity sensor. Theother mobile device understands that a logic ‘0’ is being transmitted.

In another embodiment, data control 120 contains a memory device whichcan hold a byte or word of data from registers 106. Data control 120reads or receives a byte or word of data from registers 106 and togglesswitch 122 accordingly to transmit the data one bit at a time. Once thebyte or word is transmitted, data control 120 reads or receives anotherbyte or word from registers 106 if more data needs to be transmitted.

Switch 122 is an N-channel MOSFET with a drain terminal coupled tosensing element 58, a source terminal coupled to ground node 124, and agate terminal coupled to data control 120. The gate terminal of switch122 is a control terminal of switch 122, and controls current betweenthe source and drain terminals. The source and drain terminals of switch122 are conduction terminals. In other embodiments, other types ofelectrical switches are used. When data control 120 outputs a logic ‘1’,i.e., a positive voltage, to the gate terminal of switch 122, aninversion layer is created within the N-channel MOSFET which creates aconductive channel for current between sensing element 58 and groundnode 124. Sensing element 58 is detected by another mobile device, and alogic ‘1’ is transmitted via capacitive NFC. When data control 120outputs a logic ‘0’, i.e., a voltage approximately equal to groundpotential, the channel in the N-channel MOSFET of switch 122 is closed,reducing the ability of electrical current to flow between ground node124 and sensing element 58. Sensing element 58 does not receive asignificant amount of charge that is opposite the charge on anothermobile device's sensing element. Sensing element 58 does notsignificantly impact the self-capacitance of another mobile device'ssensing element. The other mobile device receives a logic ‘0’ due to notdetecting sensing element 58.

FIG. 8a illustrates the parts of capacitive touch controllers used whenmobile device 10 transmits data to mobile device 54 via capacitive NFC.In other embodiments, mobile device 10 communicates with other devicesover capacitive NFC, such as retail point-of-sale (POS) systems,advertising kiosks, security systems, motor vehicles, workout equipment,home automation equipment, and other devices equipped with a capacitivetouch controllers similar to capacitive touch controllers 56 and 130.

CPU 34 initiates a data transfer. In one embodiment, CPU 34 controls thetiming and data rate of capacitive NFC transmission, as well ashandshaking with mobile device 54. When CPU 34 controls timing, CPU 34first writes to a bit in registers 106 to switch MUX 110 and connectdata output 108 to sensing element 58. Once data output 108 is connectedto sensing element 58, CPU 34 controls switch 122 by writing to anotherbit in registers 106. CPU 34 connects sensing element 58 to ground node124 by writing a logic ‘1’ to the bit, and puts sensing element 58 inhigh impedance by writing a logic ‘0’ to the bit. In other embodiments,capacitive touch controller 56 handles timing, data rate, andhandshaking with capacitive touch controller 130 of mobile device 54.CPU 34 writes a byte or word of data to registers 106 and capacitivetouch controller 56 transmits the data.

Registers 106 include memory elements used to store data fortransmission via capacitive NFC. CPU 34 writes a bit, byte, word, ormultiple bytes or words, to the memory elements in registers 106 fortransmission. In one embodiment, CPU 34 writes a byte to registers 106.Data output 108 transmits the byte using sensing element 58, and thenCPU 34 is interrupted to send another byte to transfer. In anotherembodiment, registers 106 include a FIFO which CPU 34 writes to untilthe FIFO is full. Data output 108 transmits the data located in the FIFOin the same order which CPU 34 wrote the data. If the FIFO becomes full,CPU 34 is interrupted when the FIFO is no longer full to continuewriting data.

Data output 108 reads a piece of data from registers 106 which instructsdata control 120 how switch 122 is to be operated. In embodiments whereCPU 34 handles data rates and handshaking, the register bit in registers106 which CPU 34 uses to control sensing element 58 is coupled to thegate of switch 122. In embodiments utilizing a FIFO in registers 106,data output 108 receives a signal from the FIFO indicating data has beenreceived from CPU 34. Data output 108 pops the next piece of data to betransmitted off the FIFO and stores the data into data control 120.

Data control 120 operates switch 122 as necessary to transmit data overcapacitive NFC using sensing element 58. In some embodiments, datacontrol 120 is a pass-through connecting a gate of switch 122 to a bitin registers 106 controlled by CPU 34. In other embodiments, datacontrol 120 contains a multi-bit memory element used to temporarilystore the next piece of data to transmit. Data control 120 updates thestate of switch 122 for each bit using timing which mobile device 54expects to properly receive the data. Switch 122 switches sensingelement 58 between being coupled to ground to transmit a logic ‘1’ andbeing high impedance to transmit a logic ‘0’.

Mobile device 10 and mobile device 54 are placed close to each otherwith sensing element 58 in proximity to sensing element 88. When sensingelement 58 is coupled to ground node 124, sensing element 58 has asignificant effect on the self-capacitance of sensing element 88, andproximity is detected by capacitive touch controller 130. When sensingelement 58 is high impedance, sensing element 58 does not have asignificant impact on the self-capacitance of sensing element 88, and noproximity is detected.

AFE 132 measure the self-capacitance of sensing element 88, and outputsa digital value approximately proportional to the capacitanceattributable to sensing element 58 to digital processing unit 134.Digital processing unit 134 compares the digital value from AFE 132 to athreshold value in registers 136 to determine whether proximity isdetected or not, i.e., whether a logic ‘1’ is being transmitted bymobile device 10, or a logic ‘0’. In one embodiment, digital processingunit 134 sets a proximity status bit in registers 136. CPU 140 isinterrupted to read the proximity status, which contains the bittransmitted by mobile device 10. CPU 34 collects individual bits fromthe proximity status bit in registers 136 and concatenates the bits intobytes, words, or other data structures for further processing by theCPU. In another embodiment, digital processing unit 134 stores the bitsas bytes or words in registers 136, and CPU 34 is interrupted when anentire data structure is received.

FIG. 8b illustrates a method of transmitting data from mobile device 10to mobile device 54 one bit at a time using capacitive NFC. In step 150,mobile device 10 initiates the transmission of a bit of data by CPU 34writing the bit to be transferred to a hardware register in registers106. In one embodiment, CPU 34 waits a predetermined period of timebefore writing a bit to registers 106 to ensure that a previouslytransmitted bit is received by mobile device 54. The period of time towait between transmitting two bits is set by a capacitive NFC standardor negotiated by mobile device 10 and mobile device 54 prior tobeginning data transmission. In other embodiments, CPU 34 writes morethan a single bit to registers 106 at once.

In step 152, data control 120 reads or receives the bit of data fromregisters 106 and controls switch 122 to connect sensing element 58 toground node 124 or high impedance. If CPU 34 wrote a logic ‘0’ toregisters 106, data control 120 opens switch 122 to put sensing element58 in a high impedance state. When sensing element 58 is set to highimpedance, sensing element 58 does not have a significant effect on theself-capacitance of sensing element 88, and mobile device 54 does notdetect proximity. If CPU 34 wrote a logic ‘1’ to registers 106, switch122 is closed and sensing element 58 is coupled to ground node 124. Whensensing element 58 is coupled to a ground potential, sensing element 58does have a significant impact on the self-capacitance of sensingelement 88. Mobile device 54 detects proximity.

In one embodiment, an output of the physical memory device in registers106 which is used by CPU 34 to transmit a bit of data is coupled to thegate of switch 122. CPU 34 directly controls the state of switch 122 bywriting to registers 106. In embodiments where CPU 34 writes multipletransfer bits at a time to registers 106, data control 120 isresponsible for receiving multiple bits from registers 106 and sendingeach bit one at a time according to the protocol being used betweenmobile device 10 and mobile device 54.

In step 154, capacitive touch controller 130 performs proximitydetection. AFE 132 operates similarly to AFE 102, cancelling theenvironmental self-capacitance of sensing element 88 to leave theportion of self-capacitance attributable to sensing element 58, Cuser.AFE 132 includes an analog to digital converter which outputs a digitalvalue proportional to Cuser to digital processing unit 134. Digitalprocessing unit 134 operates similarly to digital processing unit 104,and compares Cuser to a threshold configured in registers 136 by CPU 140to determine whether proximity is sensed or not. If Cuser is greaterthan or equal to the threshold, capacitive touch controller 130 detectsproximity and a logic ‘1’ is written to the proximity status bit. IfCuser is less than the threshold, proximity is not detected and a logic‘0’ is written to the proximity status bit. CPU 140 is interrupted whenthe new proximity status is stored.

In one embodiment, CPU 140 configures capacitive touch controller 130for an increased proximity detection frequency for a higher datatransfer rate. In some embodiments, CPU 140 configures capacitive touchcontroller 130 into a data receive mode. Capacitive touch controller 130does not interrupt CPU 140 each time proximity is read, but insteadcollects bits from proximity readings into bytes, words, or otheramounts of data as configured by CPU 140. When an entire byte or word iscollected by capacitive touch controller 130, CPU 140 is interrupted toread the byte or word instead of a single bit.

In step 158, CPU 140 reads the memory mapped proximity flag in registers106. CPU 140 is in a state expecting to receive data, and correctlytreats the proximity status flag as the next bit of data from mobiledevice 10. CPU 140 collects the data bits received from mobile device 10into bytes, words, or other data structures usable by CPU 140.

FIG. 9a illustrates transmit (TX) FIFO 162 used by mobile device 10 totransmit data to another device via capacitive NFC. TX FIFO 162 isillustrated as holding 6 pieces of data, with each piece of data beingone byte. In other embodiments, TX FIFO 162 is smaller or larger. In oneembodiment, TX FIFO 162 holds 1024 pieces of data, with each piece ofdata being a 32-bit word. CPU 34 initiates a transfer to another mobiledevice by writing a byte to TX FIFO 162. Data output 108 is notifiedthat TX FIFO 162 contains data by a FIFO empty flag from the TX FIFObeing a logic ‘0’. Data output 108 reads a byte of data from TX FIFO 162and transmits the data one bit at a time by toggling sensing element 58between ground and high impedance.

While data output 108 is busy transmitting the first byte written by CPU34, the CPU continues to write data to TX FIFO 162. CPU 34 continueswriting data to be transmitted to TX FIFO 162 until the TX FIFO containsthe next 6 bytes to be transmitted. When TX FIFO 162 contains 6 bytes tobe transmitted, the TX FIFO is full. CPU 34 is notified that TX FIFO 162is full by a FIFO full flag which becomes a logic ‘1’. CPU 34 continuesto write data to be transmitted when the FIFO full flag returns to alogic ‘0’. In some embodiments, the FIFO full flag returns to a logic‘0’ when one byte is available in TX FIFO 162. In other embodiments, CPU34 configures capacitive touch controller 56 with a specific number ofbytes which are available in TX FIFO 162 when the CPU is interrupted.

TX FIFO 162 includes two address pointers, one address pointer forwrites by CPU 34 and one address pointer for reads by data output 108.When mobile device 10 is reset, the read pointer and write pointer areset to address 0, and the FIFO empty flag is a logic ‘1’. CPU 34 writesto a single address in capacitive touch controller 56, and the datawritten goes to the address in TX FIFO 162 indicated by the writepointer. When CPU 34 writes a byte to TX FIFO 162, the write pointer isincremented to address 1 while the read pointer remains at address 0.The FIFO empty flag is set to a logic ‘0’. When data output 108 readsthe byte from TX FIFO 162, the read pointer increments to address 1, andthe write pointer remains at address 1. The FIFO empty flag is set to alogic ‘1’.

As CPU 34 continues to write data into TX FIFO 162, the write pointerincrements for each byte written. After CPU 34 writes to address 5, thewrite pointer returns to address 0 and the CPU writes the next byte toaddress 0. When CPU 34 writes to address 0, the FIFO full flag to CPU 34is asserted. The read pointer has remained at address 1, indicating thataddress 1 still contains data to be transmitted. The FIFO full flag isset so that CPU 34 does not overwrite the data in address 1 which hasnot been transmitted. As data output 108 continues to read data from TXFIFO 162 and transmit the data via sensing element 58, the read pointerincrements for each byte read from the TX FIFO. Eventually, the readpointer catches up to the write pointer and all pending data has beentransmitted. The FIFO empty flag is asserted to alert data output 108that no more data is available to be transmitted.

FIG. 9b illustrates receive (RX) FIFO 164 used by capacitive touchcontroller 56 to collect data received over capacitive NFC. In someembodiments in which data is transmitted in one direction at a time, thesame FIFO is used for RX FIFO 164 and TX FIFO 162. The FIFO isreconfigured between an RX and TX mode by changing which module iswriting to the FIFO and which module is reading from the FIFO. RX FIFO164 is illustrated as holding 6 pieces of data, with each piece of databeing one byte. In other embodiments, RX FIFO 164 is smaller or larger.In one embodiment, RX FIFO 164 holds 1024 pieces of data, with eachpiece of data being a 32-bit word. A data transfer is initiated by CPU34 setting capacitive touch controller 56 into a data receive mode. Eachsubsequent bit is read, one at a time, by capacitive touch controller 56performing a capacitive proximity reading. In one embodiment, CPU 34configures automatic periodic proximity detection at the same rate atwhich another device is configured to send data.

To detect proximity, AFE 102 generates a digital value approximatelyproportional to Cuser, i.e., the portion of the self-capacitance ofsensing element 58 attributable to another device. Digital processingunit 104 receives the digital Cuser value from AFE 102, and compares theCuser value to a threshold value to determine whether the transmittingdevice is sending a logic ‘1’ or a logic ‘0’. If the Cuser value isequal to or greater than a threshold for detecting proximity, proximityis detected and a logic ‘1’ is received. If the Cuser value is less thana threshold for detecting proximity, no proximity is detected and alogic ‘0’ is received.

Digital processing unit 104 collects the received bits in a temporaryregister until a full byte has been received. When the temporaryregister is full with 8 bits of received data, the received byte iswritten to RX FIFO 164. Digital processing unit 104 continues to receivedata and store the data in RX FIFO 164 until the RX FIFO asserts theFIFO full flag to digital processing unit 104. If RX FIFO 164 is full,storing additional data in the RX FIFO overwrites existing data. CPU 34transmits a message to the other device to stop sending additional data,and to resend any data which was transmitted between RX FIFO 164becoming full and the stop message being received by the other device.

When received data is available in RX FIFO 164, a FIFO IRQ is assertedto CPU 34 to alert the CPU that data is available to be read from the RXFIFO. CPU 34 reads bytes from RX FIFO 164 until the RX FIFO is empty,and performs any operation necessary on the received data. RX FIFO 164operates similarly to TX FIFO 162, with a write pointer incremented eachtime digital processing unit 104 stores a received byte in the RX FIFO,and a read pointer which is incremented each time CPU 34 reads a byte.

Capacitive NFC using proximity sensors commonly found in mobile devicesallows secure transfer of data between multiple devices in closephysical proximity. Utilizing proximity sensors which are alreadycommonly used in mobile devices reduces the expense and complexity ofmanufacturing mobile devices, and also reduces battery usage, comparedto existing inductive NFC. Two mobile devices, or one mobile device andanother type of device, are placed close together with sensor elementsfacing each other. The receiving side performs successive capacitivesensing measurements while the transmitting side toggles the sensingelement between a ground potential and high impedance, which generatesthe bitstream to be sent. To get a higher bitrate, the receiving sideperforms faster capacitance measurements while the transmitting sidetoggles the sensing element between ground potential and high impedancefaster.

While one or more embodiments of the present invention have beenillustrated in detail, the skilled artisan will appreciate thatmodifications and adaptations to those embodiments may be made withoutdeparting from the scope of the present invention as set forth in thefollowing claims.

1. A method of making a mobile device, comprising: providing a conductive element; providing a multiplexer with the conductive element, coupled to a first terminal of the multiplexer; providing a ground node; providing a switch coupled between the ground node and a second terminal of the multiplexer; providing a first memory element configured to control the switch; and providing a capacitive touch controller coupled to a third terminal of the multiplexer and configured to measure a capacitance of the conductive element.
 2. The method of claim 1, further including configuring the conductive element to be detected by a proximity sensor of a second mobile device.
 3. The method of claim 1, wherein providing the first memory element includes providing a data output configured to receive a byte of data and output the byte of data one bit at a time by toggling a control terminal of the switch.
 4. (canceled)
 5. The method of claim 1, further including providing a digital processing unit configured to convert the capacitance of the conductive element to a bit of data.
 6. The method of claim 5, further including providing a second memory element configured to store the bit of data.
 7. A method of making a mobile device, comprising: providing a conductive element configured to allow a second mobile device to detect the conductive element; providing a ground node; providing a switch coupled between the conductive element and ground node; and providing a first memory element configured to transmit a bit of data by toggling the switch.
 8. The method of claim 7, further including configuring the conductive element to be detected by a proximity sensor of the second mobile device.
 9. The method of claim 7, wherein providing the first memory element includes providing a register bit coupled to a control terminal of the switch.
 10. The method of claim 7, wherein providing the first memory element includes providing a FIFO.
 11. The method of claim 7, further including providing a digital processing unit configured to convert a value of a capacitance of the conductive element to a bit of data received from a second mobile device.
 12. The method of claim 11, further including providing a second memory element configured to store the bit of data.
 13. A method of making a mobile device, comprising: providing a conductive element; providing a ground node; and providing a switch coupled between the conductive element and ground node with the switch configured to put the conductive element in a high impedance state.
 14. The method of claim 13, further including configuring the conductive element to be detected by a proximity sensor of a second mobile device.
 15. The method of claim 13, further including providing a register bit coupled to a control terminal of the switch.
 16. The method of claim 13, further including: providing a data output configured to control the switch; and providing a FIFO configured to provide data to the data output.
 17. The method of claim 13, further including providing a capacitive touch controller configured to successively measure a capacitance of the conductive element.
 18. The method of claim 17, further including providing a digital processing unit configured to convert the successive capacitance measurements of the conductive element to successive bits of data.
 19. The method of claim 18, further including providing a memory element configured to store the successive bits of data.
 20. A mobile device, comprising: a conductive element; a multiplexer including a first terminal of the multiplexer coupled to the conductive element; a ground node; a switch coupled between the ground node and a second terminal of the multiplexer; and a capacitive touch controller coupled to a third terminal of the multiplexer.
 21. The mobile device of claim 20, further including a register bit coupled to a control terminal of the switch.
 22. The mobile device of claim 20, further including a data output configured to control the switch.
 23. The mobile device of claim 22, further including a FIFO configured to provide data to the data output.
 24. The mobile device of claim 20, wherein the capacitive touch controller is configured to measure a capacitance of the conductive element.
 25. The mobile device of claim 24, further including a digital processing unit configured to convert the capacitance of the conductive element to a bit of data.
 26. The method of claim 1, further including configuring the multiplexer to couple the conductive element to the switch in a transmit mode of the mobile device and to the capacitive touch controller in a receive mode of the mobile device. 